As a result of the tremendous and continuous increase in data-intensive applications, the demand for bandwidth in communication systems has been ever-increasing. In response, the installed capacity of telecommunications operations has been largely supplanted by optical fibers that provide a significant bandwidth enhancement over the traditional copper wire-based systems.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunications industry: optical amplifiers and wavelength division multiplexers (WDMs). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDMs enable different wavelengths of light to carry different signals in parallel over the same fiber. In most WDM systems there is a trade-off between the number of channels the system accommodates and the separation between adjacent channels. Higher bit rates generally call for an increase in channel spacing. Both goals favor a wide operating spectrum, that is, a wide range in operating wavelengths.
Moreover, it is important to have uniform gain over the entire operating spectrum of WDM optical communication systems. This objective becomes more difficult to reach as the operating wavelength is extended to shorter wavelengths (S-band systems, wavelengths from 1460–1530 nm), where conventional amplification techniques based on erbium-doped fiber amplifiers are unavailable. New types of optical fiber amplifiers have been developed that operate using stimulated Raman scattering. The most prominent of these is a distributed amplifier that operates over the normal transmission span as a traveling wave amplifier. Raman scattering is a process by which light incident on a medium is converted to light at a lower frequency (Stokes case) than the incident light. An optical pump source is used, where the pump photons excite the molecular vibrations of the optical medium up to a virtual level (non-resonant state). The molecular state quickly decays to a lower energy level, emitting a signal photon in the process Since the pump photon is excited to a virtual level, Raman gain can occur for a pump source at any wavelength, including the S-band (as defined above) and L-band (wavelengths approximately 1565–1625 nm). The difference in energy between the pump and signal photons is dissipated by the molecular vibrations of the host material. These vibrational levels determine the frequency shift and shape of the Raman gain curve. The frequency (or wavelength) difference between the pump and the signal photon is thus defined as the Stokes shift. The maximum Raman gain occurs at a Stokes shift of 13.4 THz (i.e., 13.4×1012 Hz), which is approximately 100 nm from Raman pumps in the optical communications window.
Since Raman scattering can occur at any wavelength, this phenomenon can be exploited to advantage in a telecommunication system that contains multiple signal wavelengths by using Raman pump sources at several different wavelengths to amplify the information signals. The gain seen by a given information signal wavelength is therefore the superposition of the gain elements provided by all of the pumps, taking into account the transfer of energy between the pumps themselves due to Raman scattering. By properly weighting the power provided at each of the Raman pump wavelengths, it is possible to obtain a signal gain versus wavelength profile in which there is a small difference between the gain seen by different information signal wavelengths (where this difference is termed “gain ripple” or “gain flatness”). The use of Raman amplification with multiple pumps thus enables dense WDM technology to be responsible for the evolution from 10 to 40 Gb/s transmission, since it improves the optical signal-to-noise ratio (OSNR) at lower launch powers.
One persistent problem with the use of multiple pumps is the unwanted nonlinear effect referred to as four-wave mixing (FWM). In general, if two intense waves (e.g., a Raman pump and an information signal, or two Raman pumps) undergo four-wave mixing, they will generate two new frequency components such that all four waves will be equally spaced in frequency. It has been found that the strength of this unwanted effect can be significantly reduced by increasing the fiber dispersion at the mixing wavelengths (see, for example, U.S. Pat. No. 5,327,516 issued to A. R. Chraplyvy et al. describing the use of non-zero dispersion to suppress FWM between multiple signals). By adjusting the location of the zero dispersion wavelength (hence, the waveguide dispersion) of the fiber, FWM can be controlled and, in many cases, essentially eliminated. In general, it is desirable to have the “zero” of dispersion at a wavelength shorter than the shortest wavelength pump, so that the dispersion is greater than approximately 1 ps/km-nm over the entire region of Raman pumps and information signals. The precise dispersion value required depends on the fiber effective area and signal channel spacing, as well as other system design details.
The inefficiency problems associated with Raman amplification have been found to be particularly severe for S-band Raman amplification, where one or more pumps needs to be very close to, or even on top of, the 1385 nm “water peak” to make use of the full S-band (since the Raman pump is generally 100 nm lower than the information signal wavelength). The well-known water peak at 1385 nm is defined as the optical loss at this wavelength as a function of the water remaining in the glass. The more water that is present, the higher the loss. Accordingly, hydroxyl-ion absorption is frequently referred to as “water” absorption, and arises from lightwave energy being absorbed by the OH ion at wavelengths that are related to its different vibration modes. For example, the two fundamental vibrations of this ion occur at 2730 and 6250 nm, corresponding to its stretching and bending motions, respectively. Nevertheless, overtones and combination vibrations strongly influence the loss in the near infrared and visible wavelength regions. In particular, as mentioned above, the overtone at 1385 nm resides in the heart of region required for S-band Raman amplification. Indeed, concentrations of OH in the fiber core as low as one part per million (ppm) have been found to causes losses as high as 65 dB/km at 1385 nm. It is desirable to reduce this OH concentration to a level such that the overall optical loss at 1385 nm is at least comparable to the overall optical loss at, for example, 1310 nm (approximately 0.325 dB/km for matched clad fiber). It is currently commercially feasible to maintain the OH concentration at substantially less than one part per billion (ppb), particularly if VAD processing is used to make the core. However, the more complex the index profile becomes beyond simple matched clad designs, the more difficult it becomes to keep the OH concentration consistently below the sub-ppb level.
As mentioned above, Raman pumps are placed approximately 100 nm below (to the “blue” side of) the signal wavelength. As a result, operation in the lower two-thirds of the S-band suffers greatly from the presence of the water peak attenuation centered at 1385 nm. In particular, the Raman gain in the S-band (using one first-order pump) can be expressed as:G=exp(CR*Ppump*Leff), whereLeff=[1−exp(−αpump*Lspan)]/αpump,CR is defined as the Raman gain coefficient and αpump is the loss at the pump wavelength. It is possible to compensate for fiber loss simply by raising the Raman pump power, if the goal is only to match the span Raman gain. This, however, may increase the cost of Raman pumping, as well as increase the heat dissipation load. Furthermore, if two spans with differing fiber loss are pumped to equal Raman gain, the span with higher loss will suffer more degradation of optical signal to noise ratio (OSNR) than the span with lower loss. Thus, higher fiber loss will either impair transmission performance or add system cost. In addition, variability in pump region loss is a severe problem for system engineers, since it is not known a priori what the loss (and thus the Raman gain) will be for a deployed fiber span.
Since it will be desirable, in future systems, to use Raman amplification in the S-band of optical signal transmission, it is necessary to develop a set of fiber parameters that overcome the problems of FWM and water peak attenuation in the short wavelength S-band regime.